The Science Case for Spacecraft Exploration of the Uranian Satellites Lead Authors: Richard J. Cartwright1 & Chloe B. Beddingfield1,2 1 2 SETI Institute, NASA Ames Research Center [email protected] [email protected] Uranian satellites imaged by Voyager 2, not shown to scale (NASA/JPL-Caltech/USGS, [55]). Puck (top left), Miranda (top middle), Ariel (top right), Umbriel (bottom left), Titania (bottom middle), and Oberon (bottom right). Co-Authors: T. Nordheim, Jet Propulsion Laboratory D. Burr, Northern Arizona University C. Elder, Jet Propulsion Laboratory A. Ermakov, University of CA Berkeley W. Grundy, Lowell Observatory J. Roser, SETI Institute & NASA Ames B. Buratti, Jet Propulsion Laboratory J. Castillo-Rogez, Jet Propulsion Laboratory A. Bramson, Purdue University M. Showalter, SETI Institute M. Sori, Purdue University I. Cohen, John Hopkins University, APL R. Pappalardo, Jet Propulsion Laboratory E. Turtle, John Hopkins University, APL M. Neveu, Goddard Space Flight Center M. Hofstadter, Jet Propulsion Laboratory Additional Co-Authors and Endorsers: A full list of co-authors and endorsers is included at the end of this document. 1. Introduction and Motivation A spacecraft mission to the Uranian satellites would address the following ‘Big Questions’ identified in the Scientific Goals for Exploration of the Outer Solar System document, outlined by the OPAG community (https://www.lpi.usra.edu/opag/goals-08-28-19.pdf) [Table 1]: (1) What is the distribution and history of life in the Solar System? (2) What is the origin, evolution, and structure of planetary systems? (3) What present-day processes shape planetary systems, and how do these processes create diverse outcomes within and across different worlds? The large moons of Uranus are possible ocean worlds [1] that exhibit a variety of surface features , hinting at endogenic geologic activity in the recent past (e.g., [2]). These moons are rich in water ice, as well as carbon-bearing and likely nitrogen-bearing constituents, which represent some of the key components for life as we know it. However, our understanding of Uranus and its moons is severely limited by the absence of data collected by an orbiting spacecraft. We assert that multiple close proximity flybys of the Uranian moons made by a Flagship-class spacecraft in orbit around Uranus is needed to conduct essential Solar System science, and initiation and design of this mission must occur in the upcoming decade (2023 – 2032). An orbiter would vastly improve our understanding of these possible ocean worlds and allow us to assess the nature of water and organics in the Uranian system, thereby improving our knowledge of these moons’ astrobiological potential. A Flagship mission to Uranus can be carried out with existing chemical propulsion technology by making use of a Jupiter gravity assist in the 2030 – 2034 timeframe, leading to a flight time of only ~11 years, arriving in the early to mid 2040’s (outlined in the Ice Giants Pre-Decadal Survey Mission Study Report: https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf). Crucially, this arrival timeframe would allow us to observe the Uranian moons’ northern hemispheres, which were shrouded by winter at the time of the Voyager 2 flyby and have never been imaged. An orbiter could then continually collect data and observe seasonal changes to the surfaces of these moons as the Uranian system transitions into southern spring in 2049. A complementary assessment of the science that could be achieved by a Flagship mission to the Uranus system is described in another paper submitted to the Planetary Science and Astrobiology Decadal Survey [3]. The five large moons of Uranus are enigmatic, with surfaces rich in volatiles and marked by bizarre landforms, hinting at geologically complex and recent activity. In 1986, the Voyager 2 spacecraft flew by the Uranian system and collected tantalizing snapshots of these ‘classical’ satellites, measured Uranus’ offset and tilted magnetic field, as well as discovering ten new ring moons (e.g., [2]). Since this brief flyby, investigation of Uranus and its rings and satellites has remained in the purview of ground- and space-based telescopes. Although these telescope observations have made some fascinating discoveries, many key science questions remain unanswered [Table 1]. Addressing these questions is vital for a fuller understanding of the Uranian system, which represents the highest unaddressed priority item from the last Planetary Science Decadal Survey (2013 – 2022). New measurements made by modern instruments on board an orbiting spacecraft are critical to investigate the surfaces and interiors of the large moons and determine whether they are ocean worlds with subsurface liquid water layers. Furthermore, a spacecraft mission to Uranus would enable a more complete investigation of organics and water in the outer Solar System, two of the key components for life as we know it, as well as improve our understanding of how geologic processes operate in cold and distant ice giant systems. 1 Table 1: Science questions requiring exploration by a Flagship-class Uranus orbiter. SCIENCE QUESTIONS MEASUREMENTS INSTRUMENTS Do the satellites have subsurface oceans Search for induced magnetic Magnetometer that are, or were, harbors for life? Are fields, plumes, hot spots, VIS camera there signs of communication between cryovolcanic features, and surface Mid-IR camera their surfaces and interiors? Are any of changes since the Voyager 2 flyby, VIS/NIR mapping these moons geologically active? and search for dust samples from spectrometer Addresses OPAG Big Questions #1, 2, 3 possible plume sources Dust spectrometer What are the internal structures of the Moment of inertia measurements, Radio science classical satellites? gravity field characterization, subsystem Addresses OPAG Big Questions #2, 3 analysis of geologic, topographic, VIS camera spectral maps, and magnetic VIS/NIR mapping induction spectrometer Magnetometer What processes modify the satellites Analysis of geologic, topographic, VIS camera and what are the compositions of their and spectral maps, estimate surface VIS/NIR mapping geologic units and features? ages from impact crater densities spectrometer Addresses OPAG Big Questions #2, 3 Do the moons have tenuous atmospheres? Search for exospheres and changes VIS camera Do volatiles migrate seasonally? in the distribution and spectral VIS/NIR mapping Addresses OPAG Big Questions #2, 3 signature of condensed volatiles spectrometer Plasma spectrometer Do magnetospheric charged particles Characterize magnetic field and Magnetometer weather the surfaces of the ring moons charged particle populations Plasma spectrometer and classical satellites? proximal to moons Energetic particle Addresses OPAG Big Questions #2, 3 detector Is the red material on the classical Spectral maps of classical moons, VIS camera satellites organic-rich and did it originate inbound flyby of an irregular VIS/NIR mapping from the irregular satellites? satellite, images of the irregular spectrometer Addresses OPAG Big Questions #2, 3 moons while in Uranus orbit, and Dust spectrometer collect and analyze dust samples Does Mab sustain the µ-ring? Does Spectral mapping of the ring VIS camera µ-ring material coat other moons? moons and Miranda, and collect VIS/NIR mapping Addresses OPAG Big Questions #2, 3 and analyze µ-ring dust samples spectrometer Dust spectrometer What is the dynamic history of the Eccentricities, inclinations, tidal Radio science moons? Were there previous orbital Q(ω) of Uranus, Love numbers, subsystem resonances? paleo heat fluxes VIS Camera Addresses OPAG Big Questions #2, 3 2. Geology of the Uranian Satellites Classical Moons: Voyager 2 collected fascinating images of the five large moons’ southern hemispheres (subsolar point ~81ºS) [Figure 1], but their northern hemispheres were shrouded by winter darkness at the time of the flyby. The incomplete spatial coverage, and generally low spatial resolution of the available images, limits our understanding of different terrains and geologic features, in particular for the more distant moons Umbriel, Titania, and Oberon. 2 Figure 1: Voyager 2 images of the Uranian moons. White arrows highlight: (a) ridges on Miranda, which possibly have a cryovolcanic and tectonic origin; (b) Arden Corona on Miranda with high and low albedo banding along large tectonic faults; (c) Inverness (bottom left) and Elsinore (top right) Coronae on Miranda that exhibit ridges and grooves. Between these two coronae are examples of craters that have been mantled by an unknown source of regolith; (d) large chasmata with medial grooves on Ariel; (e) an impact crater on Ariel, possibly infilled by cryolava; (f) the bright floor of Wunda crater on Umbriel; (g) the large Messina Chasmata on Titania; and (h) the smooth floor of Hamlet crater and an 11 Km tall ‘limb mountain’ on Oberon. The innermost moon Miranda displays abundant evidence for endogenic geologic activity, including three large polygonal shaped regions called coronae, which were likely formed by tectonic and/or cryovolcanic processes (e.g., [2, 4-11]) [Figure 1a-c]. The origin and time scale of activity on Miranda is not well understood, and it is unknown if this activity is associated with a subsurface ocean, either now or in the past. Investigation of induced magnetic fields, plumes, surface heat anomalies, as well as analysis of geologic surface features interpreted to be cryovolcanic is paramount to determine if Miranda is an ocean world.